Note for Pet Owners:

This information is provided by Provet for educational purposes only.

You should seek the advice of your veterinarian if your pet is ill as only he or she can correctly advise on the diagnosis and recommend the treatment that is most appropriate for your pet.

Topics on this Page:

Hypothyroidism is a condition in which there are low concentrations of thyroid hormones (triiodothyronine (T3) and thyroxine (T4) )  in the blood. It is one of the most common hormonal disorders in dogs, but rare in cats. Thyroid hormone is important for the normal regulation of metabolic rate and activity in many tissues and a deficiency of the hormone results in reduced metabolic activity and abnormal changes in many organ systems. Affected animals are often lethargic, dull, may gain weight and develop visible changes in a variety of tissues...for example in the skin there is often hairloss and a dull, dry, scurfy coat. If present from birth (ie congenital) hypothyroidism is called cretinism.

There is a formal classification system for hypothyroidism.

There are many potential causes of low thyroid hormone production including :

  • A failure of the thyroid gland to develop at all (called agenesis) ...results in congenital hypothyroidism
  • Underdevelopment of  thyroid tissue mass (hypoplasia) is a cause of primary hypothyroidism (rare)
  • Iodine deficiency during foetal development (rare) - results in congenital hypothyroidism, or in adulthood causes acquired hypothyroidism (rare)
  • Autoimmune disease in which the body produces antibodies against the body's own thyroid tissue. These antibodies destroy the normal thyroid tissue replacing it with fibrous tissue. This is called lymphocytic thyroiditis and is one of the most common cause of primary hypothyroidism in dogs. In Beagles it may be a familial disorder...but it does not lead to hypothyroidism in all individuals.
  • Degeneration of the thyroid tissue due to cell death (necrosis) or reduced tissue mass (atrophy) is also a common cause of hypothyroidism
  • Cancer of the thyroid may cause primary hypothyroidism (rare)
  • Lack of thyroid stimulating hormone (TSH) from the pituitary gland in the brain results in lack of stimulation of the thyroid to secrete it's hormones, resulting in secondary hypothyroidism - though this is uncommon (accounts for less than 5% of cases)
    • This can be due to a congenital defect in the pituitary and is associated with dwarfism because there is usually a lack of growth hormone as well 
    • This can be acquired as in cancer of the pituitary gland which results in reduced TSH secretion.
  • Lack of secretion of thyrotropin releasing hormone (TRH) from the hypothalamus in the brain will result in a lack of production of TSH in the pituitary and so a lack of production of thyroid hormones - but this tertiary form of hypothyroidism has not been reported to occur in dogs
  • Hypothyroidism can result following the surgical removal or chemical destruction or suppression of the thyroid gland as a treatment for thyroid cancer, and excess thyroid hormone secretion.. This is called iatrogenic hypothyroidism.

Breed Occurrence
Large and giant breeds of dogs are most often reported to be affected with hypothyroidism including Airedale Terriers, Akita, Beagles (lymphocytic thyroiditis), Boxers, Cocker Spaniels (American), Collie, Dachshunds, Doberman Pinschers, German Shepherd Dogs, Golden Retrievers , Irish Setters, Labradors, Miniature Schnauzers,  Old English Sheepdogs and  Shetland Sheepdogs. Some texts mention the Alaskan Malamute as having a higher risk.

Most animals are affected during middle age (4-8 years) and both sexes are equally affected.

Clinical signs of hypothyroidism in an affected individual can vary because the thyroid hormones affect many organ systems. S
ome of the most common signs include :

  • Lethargy
  • Weakness and poor exercise tolerance
  • Mental dullness
  • Weight gain
  • Hair loss (symmetrical on both sides of the body) on the flanks, neck, thighs and tail. Not usually itchy.
  • Occasionally, in some breeds eg Irish Setters,  the coat becomes thicker
  • Dull, dry, scurfy coat (dandruff). Occasionally itchy.
  • Black pigmentation and thickening of the skin.
  • Abnormal oestrus in bitches (irregular, prolonged bleeding), infertility, abortions and stillbirths, fading puppy syndrome (early post-natal deaths).
  • Abnormal lactation and breast development
  • Reduced size (atrophy) of the testicles in males, poor libido and poor sperm production
  • Low body temperature
  • Neurological disorders including facial paralysis, head tilt, laryngeal paralysis, loss of proprioception (knuckling of the feet), megaoesophagus, muscle wastage, stiffness, weakness
  • Anaemia, bleeding disorders,a low white cell count (hence a poor immune response to infections)
  • Slow heart rate and abnormal cardiac rhythm. Cardiomyopathy.
  • Constipation, and sometimes vomiting or diarrhoea
  • Aggression has been reported to be associated with atypical hypothyroidism 
  • Provet is interested in feedback from you on some of these, and other signs of hypothyroidism CLICK HERE   

Provet is conducting an on-going  survey into the relationship between hypothyroidism and these and other clinical conditions  CLICK HERE for more information

In humans enlargement of the thyroid gland (seen as a swelling in the neck below the Adam's apple - called goitre) is a common feature of hypothyroidism but this is rare in dogs, except in association with thyroid cancer.

Severe hypothyroidism can lead to myxoedema (generalised body oedema) and coma. (very rare).


Diagnosis is based upon the clinical signs and laboratory tests, in particular the measurement of blood thyroxine (T4) concentrations will confirm the presence of hypothyroidism.:

  • Basal - Normal range is 17-46 nmol/l in dogs. Unfortunately, the basal concentration of T4 can be affected by other disease situations - called the " euthyroid sick syndrome " (CLICK HERE for a list ) and so it is an unreliable measurement by itself. In addition several drugs can reduce the T4 concentrations CLICK HERE for further details.
  • Thyroid Stimulating Hormone (TSH) stimulation test. Collect starved blood sample. Administer 0.1 IU TSH/ kg body weight IV. After 6 hours take second sample. Measure T4 in both samples. Hypothyroid dogs show little or no increase. Dogs with "euthyroid sick syndrome", or with drug interference show an increase in T4 in response to the TSH.

Other non-specific abnormal laboratory findings which may be present in hypothyroid patients include :

  • Anaemia (normocytic, normochromic, non-regenerative)
  • High blood cholesterol
  • High blood triglyceride
  • High blood creatine kinase
  • Increased ALT, AST, ALT and LD
  • Proteinuria - if auto-immune damage to kidney as well as thyroid

The main method of treatment for hypothyroid dogs is thyroid hormone replacement. Treatment with thyroid replacement therapy should only be started once any concurrent disease (eg diabetes or cardiac disease) has been stabilised. Also, the dose rate should be reduced to a quarter if heart disease is present.

Usually a synthetic thyroxine (T4) drug is given by mouth at a relatively high dose rate : 20mg/kg body weight twice daily. 

Synthetic triiodothyronine (T3) can also be given  Dose 5mg/kg body weight three times daily initially reducing to twice daily.

Dried thyroid tablets are NOT recommended for use in dogs because the active ingredient dose is very variable.

Dietary management involves a weight-reduction diet followed by maintenance on a relatively low-calorie ration. the aim is to get the animals body weight back to normal and maintain it. Calorie intake needs to be increased if the patient loses too much weight whilst on treatment.

Stabilising hypothyroid patients can be tricky, especially if other disorders are present at the same time. 

Side effects are uncommon.

Because thyroid supplementation increases metabolic rate and activity in many tissues patients should be checked regularly (every 6 months) to ensure there are no signs of overdosage.

The prognosis for most cases is good once they are stabilised on thyroid therapy and providing other concurrent major organ diseases are stable.

Long term problems
Atherosclerosis is rare in dogs, but it has been reported to occur in dogs with untreated hypothyroidism, and this disease can create serious circulatory difficulties for the patient.


Updated October 2013